Field of the Invention
This invention relates to a dosimeter for monitoring workers and working areas and in particular a dosimeter having a low air flow rate.
Dosimeters are known and have been used in effort to determine the level of exposure of workers to foreign substances in air, for example, to chemical vapors or fumes, dust particles and the like. A dosimeter is placed in a work area or worn by an individual and air is pumped through a filter which traps foreign substances in the air. At the end of an exposure period, the filter is removed and analyzed for any foreign substances. Such dosimeters are shown in U.S. Pat. No. 4,063,824 issued Dec. 20, 1977 to Baker and Clark and U.S. Ser. No. 800,430 filed May 25, 1977.
One problem with such dosimeters is that a dosimeter designed to utilize a high flow rate of air such as 500-4000 cubic centimeter per minute (cc/min.) cannot be operated at a low flow rate such as 25-125 cc/min. Internal friction of the pump in the dosimeter when operated at low speeds to provide low air flow rates causes the pump to operate irregularly. High air flow rates are desired for dosimeters that use filters to measure dust particles in the air. Low air flow rates are desired for dosimeters that use filters such as charcoal filters to measure vapors or fumes in the air. Dosimeters containing pumps designed to operate at low air flow rates cannot be operated at high air flow rates.
Instead of having several dosimeters that operate at different air flow rates, there is a need for a single dosimeter designed in such a manner that both high and low air flow rates are provided and wherein the air flow rates are accurate, uniform and controlled.